This invention relates to organic electroluminescent (EL) devices. More particularly, the invention relates to the use of a single organic fluorescent material to obtain white emission or variable colors.
Since Tang and Vanslyke have made the first thin film organic light-emitting diode by vacuum deposition (see Appl. Phys. Lett. Vol. 51, 1987, P. 913), there has been considerable interest in the use of organic materials for fabrication of organic light-emitting diodes (LEDs). As a result, more and more new materials, processing technologies, and cell configurations have been developed to improve the performance of the organic LEDs and obtain various emission color, especially white color for back-light and full color applications. Together with its wide viewing angle, high contrast, high brightness, and potentially low production cost, organic LEDs have a good potential for large-area flat panel display applications.
In a fundamental organic LED structure, one organic layer is specifically chosen to inject and transport holes and the other organic layer is specifically chosen to inject and transport electrons. The interface between the two layers provides an efficient site for the recombination of the injected hole-electron pair and resultant electroluminescence. The simple structure can be modified to a three-layer structure, in which an additional luminescent layer is introduced between the hole and electron transporting layer to function primarily as the site for hole-electron recombination and thus electroluminescence. In this respect, the functions of the individual organic layers are distinct and can therefore be optimized independently. Thus, the luminescent or recombination layer can be chosen to have a desirable EL color as well as a high luminance efficiency. Recently devices have been made with various configurations by inserting a luminescent layer, especially a multiluminescent layer with different luminescent materials, to obtain white emission or variable colors.
White organic LEDs are very important in full color displays and for back-lights of LCDs. In order to obtain white emission, the three basic colors red, green and blue need to be mixed together. Several methods have been employed to achieve white emission in organic LEDs. For example, multi emission layers may replace the single emission layer, with red, green, and blue emissions coming from the different emission layers, or RGB-emitting materials may be doped into a single polymer. In practice, however, it is very difficult to control the intensity ratio of the each RGB peak to obtain white emission. Furthermore, devices using the above methods will change chromaticity with the to driven voltage. Another approach for white emission is to use a white-emitting material that produces white emission by itself, but this is a substantial challenge for synthetic chemists.
It is an object of the present invention to disclose an organic LED in which single emitting material is used to obtain white emission.
It is another object of the present invention to provide a technique to obtain variable color emission in an organic EL devices by utilizing the aggregate property of the organic fluorescent materials.
According to the present invention there is provided an organic electroluminescent device comprising: a substrate formed of an electrically insulating material; an anode layer; an organic hole transporting layer; an organic electron transporting layer; an organic light emitting structure located between the hole transporting layer and the electron transporting layer, said light emitting structure comprising at least two sub-layers doped with the same fluorescent material at different concentrations; and a cathode layer.
The use of the aggregate property of organic fluorescent materials can change color of the organic EL device using single fluorescent materials. Different aggregate conditions of the fluorescent materials can emit different colors because of the interaction of the materials. As a result, using a structure for the light emitting layer in the organic EL devices which comprises different aggregates of the organic materials, white light emission can be obtained.
The substrate may be either optically transparent (eg plastics or glass) or opaque (eg semiconducting materials or ceramics). If the substrate is opaque, the anode may be transmissive and may be selected from the group consisting of a metal oxide, gallium nitride, zinc selenide and zinc sulphide. When the anode is a metal oxide it may be indium-tin-oxide, aluminium or indium doped tin oxide, magnesium-indium oxide, fluorine tin oxide, nickel-tungsten oxide and cadmium tin oxide. Conversely if the substrate is transparent, the anode may be opaque or transparent and formed of any material such as a metal or metallic compound provided that the work function is greater than 4.1 eV. Possible metals include gold, iridium, palladium and platinum.
The organic hole-transporting layer may be formed of a material including hole-transporting aromatic tertiary amine molecules.
To provide variable color properties the light emitting diode may comprise a light emitting structure comprising multiple sub-layers, each sub-layer comprising a different concentration of the fluorescent material. For example there may be from 2 to 5 sub-layers.